This invention relates to silica gel carrying a titanium oxide photocatalyst in a high concentration that is capable of deodorizing automobiles, rooms, and stockbreeding pens or detoxifying toxic pollutants, of removing NOx in the atmosphere, decolorizing waste water caused by dyeing, preventing the emergence of algae in water tanks, and sterilizing water and air, in which the titanium oxide is contained in a high concentration in the pores of the silica gel, which has an adsorption performance, and relates to a method for preparation thereof.
It has been well known that for a photocatalytic reaction of a semiconductor, when powders of a semiconductor are dispersed in a water solution to which a light having the same amount or more of energy as that of the band gap of the semiconductor (light having a wavelength of 400 nm or less) is made to irradiate, electrons and electron holes generated by optical pumping are transferred onto the surface of semiconductor particles and act on an ionic species and a molecular species in the water solution to cause various reactions, such as the decomposition of water. Titanium oxide is exemplified as a representative photocatalyst.
By irradiating a semiconductor by light such as sunlight, or the light of a fluorescent tube, incandescent lamp, black light, lamp, metal-halide lamp, or a cold cathode fluorescent tube, the decomposition and removal of environmental pollutants can be conducted, such as the decomposition and removal of smelly and toxic substances in the air, the treatment of waste water, a water-purifying treatment, or the sterilization of microorganisms in water.
Due to the difficulty of handling fine powders for those uses, suggested in Japanese Patent Early-publication No. 6-65012 is a titanium oxide film photocatalyst that is excellent in water-resistant and heat-resistant properties and in durability. It is obtained by producing a sol of titanium oxide from an alkoxide of titanium, coating the sol on a glass substrate by dip coating, drying and calcining the sol-coated glass substrate to form a transparent titanium oxide film photocatalyst, or by further coating a metal on the photocatalyst by photo-electrodeposition, etc. However, it has in its use big problems, such as that this catalyst can be applied only to an inorganic substance, the shape of which is limited, such as glass, ceramics, etc., and also that since a decomposition reaction by a photocatalyst is caused only on the surface of the photocatalyst, to continuously decompose and remove environmental pollutants a very large area must be provided for the photocatalyst.
Considering these problems, the present invention has been achieved. This invention provides silica gel carrying a photocatalyst in a high concentration and a method for preparation thereof. The silica gel of this invention has an improved performance in decomposing environmental pollutants such as smelly or toxic substances in the air, or organic solvents, agricultural chemicals, etc., contained in water, by providing a concentration gradient of titanium oxide such that the concentration near the surface of the silica gel is high and the concentration of the central part is low, said silica gel also having excellent properties of safety, economic efficiency, stability, and water-resistance (even if it is put into water it does not break apart).
This invention provides silica gel carrying a titanium oxide photocatalyst in a high concentration characterized in that, to achieve the above objects, the amount of the titanium oxide contained in the pores near the surface of the silica gel is 7-70% by weight, provided that a concentration gradient is provided such that the amount is 1.5 times or more than the amount of the titanium oxide contained in the pores near the center of the silica gel, said silica gel having an average pore diameter of 6-100 nm. This invention also provides a method for the preparation of silica gel carrying a titanium oxide photocatalyst in a high concentration characterized by mixing silica gel with a titanium-containing solution in the same volume as or less than the total pore volume of the silica gel, to have the silica gel contain the titanium-containing solution, followed by heating and calcining, said silica gel having an average pore diameter of 6-100 nm.
The silica gels used in the present invention are amorphous silicon dioxides having an average pore diameter of 6-100 nm, said silica gel not breaking apart even if it is immersed in water or an organic solvent. They can be obtained by a known method such as is described in Japanese Patent Publication No. 7-64543, etc. In this method a silica hydrogel obtained by neutralizing an alkaline silicate water-solution is dried at 100-1,000xc2x0 C. by superheated steam to result in a silica xerogel, to adjust the sizes of the colloidal particles that constitute the silica gel, or by obtaining Q-6, Q-10, Q-15, Q-30, or Q-50, which are commercially available as series of CARiACT Q (produced by Fuji-Silysia Chemical Ltd.). Those silica gels greatly differ from silica gel generally used as a desiccant for packaging (an A-type or B-type standardized product under JIS Z 0701, desiccant for packaging). Since they have good mechanical strength, adsorption performance, and the property of not breaking apart even if they are immersed in water or an organic solvent, they are also used for chromatography or catalytic supports. They are applicable in various forms, from fine powders to granules. The desiccant as described above that is used for packaging has a critical defect in that when it is immersed in water or an organic solvent it crumbles. Thus it cannot be used for the present invention.
In the present invention, when the average pore diameter of the silica gel used is less than 6 nm the titanium-containing solution clogs the pores near the surface of the silica gel and thus the solution does not sufficiently impregnate the internal parts. Also, when silica gel has an average pore diameter of more than 100 nm, it is difficult to produce it and it is very expensive, and thus it is undesirable.
The crystalline structure of the titanium oxide to be contained in the pores of the silica gel of the present invention is desirably anatase, which has a high photocatalytic activity.
The silica gel carrying a titanium oxide photocatalyst in a high concentration of the present invention is one in which the amount of titanium oxide contained in pores near the surface of silica gel is 7-70% by weight, and a concentration gradient is provided such that the amount is 1.5 times or more than the amount of the titanium oxide contained in the pores near the center of the silica gel. The photocatalytic reaction of the titanium oxide occurs with an ultaviolet light having a wavelength of 400 nm or less. For an ultraviolet light having a wavelength of 380 nm, about 60% of the light is absorbed in a titanium oxide film having a thickness of 1 xcexcm, and the titanium oxide that is involved with the photocatalytic reaction is the only one present near the surface of the silica gel. Since a titanium-containing solution is very expensive, the silica gel carrying a titanium oxide photocatalyst in a high concentration of the present invention is economically very excellent. The silica gel of this invention contains titanium oxide in a high concentration in the pores near its surface, while the titanium oxide is contained in a lower concentration or none at all in the pores of the inner part, where ultraviolet light does not reach and thus no photocatalytic reaction occurs. The titanium oxide content contained in the pores near the surface of the silica gel can be determined by EPMA, which can measure to a depth of 1-2 xcexcm from the surface of the silica gel. The titanium oxide content contained in the pores near the center of the silica gel can be determined by EPMA by measuring the cross section obtained by cracking the silica gel.
The titanium oxide-containing solution used in the present invention may be, but is not limited to, organic titanium-containing solutions such as tetraisopropyl titanate, tetrabutyl titanate, butyl titanate dimer, tetrakis(2-ethylhexyloxy)titanium, tetrastearyl titanate, triethanolamine titanate, alkoxides of titanium such as diisopropoxy-bis(acetylacetonato)titanium, dibutoxy-bis(triethanolaminato)titanium, titanium ethylacetoacetate, titanium isopropoxyoctylene glycolate, titanium lactate, and titanate-based coupling agents such as isopropyltriisostearoyl titanate, isopropyltridodecylbenzenesulfonyl titanate, isopropyltris(dioctylpyrophosphate)titanate, tetraisopropylbis(dioctylphosphite)titanate, tetraoctylbis(ditridecylphosphite)titanate tetra(2,2-dialyloxymethyl-1-butyl)bis(ditridecyl)phosphite titanate, bis(dioctylpyrophosphate)oxyacetate titanate, bis(dioctylpyrophosphate)ethylene titanate, isopropyltri(dioctylphophate)titanate, isopropyltricumylphenyl titanate, and isopropyltri(N-amidoethyl-aminoethyl)titanate, and inorganic titanium-containing solutions such as titanium sulfate, titanium chloride, and titanium bromide.
Also, the above titanium-containing solution can be used alone or as a mixture of two or more of them, without limitation. It may be diluted with a solvent that is compatible therewith to adjust its concentration. As a diluent, any one that is compatible with the titanium-containing solution, such as ethanol, 1-propanol, 2-propanol, n-hexane, benzene, toluene, xylene, trichlene, propylene dichloride, and water can be used alone or as a mixture of two or more of them, without limitation.
The silica gel carrying a titanium oxide photocatalyst in a high concentration of the present invention may be obtained by adding to, e.g., a cylindrical vessel provided with a cap, the silica gel having an average pore diameter in a range of 6-100 nm as stated above and a titanium-containing solution in the same volume as or less than the total pore volume of the silica gel, rotating, vibrating, or shaking the vessel to cause the silica gel to contain a titanium-containing solution, and heating and calcining the resulting silica gel. Usually when this procedure is conducted once, a sufficient amount of titanium oxide being carried can be obtained. However, when the same procedure is conducted plural times, the amount being carried can be further increased.
When a titanium-containing solution is made to be contained in the silica gel, it is not preferable to use a conventional method such as the impregnating method, which has been generally used. This is because a titanium-containing solution in an amount greater than the total pore volume that the silica gel has is used, and thereby an excess titanium-containing solution covers the surface of silica gel, which is then calcined by heating to form a titanium oxide film that has a very small specific surface and that tends to peel. Even if a titanium-containing solution is used in an amount greater than the total pore volume of the silica gel, if an excess amount of the titanium-containing solution that covers the surface of the silica gel is removed by washing it with a diluting liquid etc., a titanium oxide film on the surface of the silica gel is not formed when heating and calcining the silica gel, and thus the above problem is overcome. However, this method is not preferable because a large amount of waste liquid caused by washing it results, and a very expensive titanium-containing solution is wasted.
To obtain a titanium oxide having a high performance as a photocatalyst, the crystalline form of which is anatase, the calcination by heating the silica gel containing a titanium-containing solution of the present invention is conducted by heating it by gradually raising the temperature of it from room temperature, maintaining the final temperature of 400-700xc2x0 C. for a certain period, followed by cooling it to room temperature. If the calcination temperature is lower than 400xc2x0 C., or higher than 700xc2x0 C., a titanium oxide results in which the rutile or amorphous form is mixed. Thus such a temperature is not preferable.
As the furnace to be used for calcination, a gas furnace that provides oxygen sufficient for the calcination is desirable. However, an electric furnace in which insufficient oxygen is provided can also be used for the calcination, without causing a problem if more oxygen is added. When the procedure of carrying titanium oxide and calcination is conducted twice, more titanium oxide can be carried.
When a titanium-containing solution in a volume less than the total pore volume of the silica gel is made to be contained in the silica gel, the solution slowly infiltrates from the surface of the silica gel into the center of it, to form hollow spheres remaining as cavities in the center of the silica gel. Heating and calcining the thus-obtained silica gel gives the silica gel carrying a titanium oxide photocatalyst in a high concentration of the present invention, with only the surface parts of it containing titanium oxide. Also, even if a titanium oxide solution is made to be contained in the same volume as the total pore volume of the silica gel, if the concentration of a diluting liquid contained in the titanium-containing solution is 5-75% by weight and if the diluting liquid has a low molecular weight, due to the filtering function of the pores of the silica gel the silica gel that is provided with a gradient in the concentration of titanium oxide in it is obtained such that the nearer the surface part of the silica gel, the higher the concentration is, and the nearer the central part of it, the lower the concentration is. The thus-obtained silica gel is heated and calcined to result in silica gel of the present invention carrying a titanium oxide photocatalyst in a high concentration provided with a gradient such that the nearer the surface part of the silica gel, the higher the concentration of titanium oxide is, and the nearer the central part of it, the lower the concentration of titanium oxide is.
When a titanium-containing solution that is readily hydrolyzable is used, the degree of the hydrolyzation of the solution is higher for silica gel having a smaller average pore diameter. However, titanium more promptly reacts with the silanol groups present on the surface of the silica gel. Thus the solution is contained on the surface part in a higher concentration. The thus-obtained silica gel of the present invention is calcined to result in silica gel carrying a titanium oxide photocatalyst in a high concentration of the present invention, said silica gel being provided with a gradient in the concentration of titanium oxide such that the nearer the surface part of the silica gel, the higher the concentration is, and the nearer the central part, the lower the concentration is.
Although the period for mixing the silica gel and a titanium oxide-containing solution varies based on the kind of titanium oxide-containing solution to be used, it is one minute or more. It is preferable that it be 30-60 minutes, because titanium can be contained in the silica gel that is provided with a more uniform concentration gradient.
Also, when by a known method at least one kind of substance selected from the transition elements of atomic numbers 21 (Sc) to 29 (Cu), 39 (Y) to 47 (Ag), 57 (La) to 79 (Au), and 89 (Ac) to 103 (Lr), and an oxide thereof is made to co-exist with the titanium oxide photocatalyst in the pores of the silica gel carrying the photocatalyst of the present invention, the photocatalyst activity may increase. Although the reason therefor is unclear, a reaction is known as Fenton""s reaction, and is such that when light is applied to titanium oxide, the resulting hydrogen peroxide, which is one kind of active oxygen, is reacted with a divalent. ferrous ion to produce a hydroxide radical. Thus it is believed that a mechanism similar to this operates with the transition elements other than iron.
The silica gel of the present invention, carrying a titanium oxide photocatalyst in a high concentration, can adsorb odors and toxic substances in the air, and organic solvents and agricultural chemicals in water in a larger amount, by containing titanium oxide in the pores of the silica gel, said pores having a larger specific surface, the silica gel being provided with a gradient in the concentration of titanium oxide. By irradiating on it light such as sunlight, or the light of a fluorescent tube, an incandescent lamp, black light, an ultraviolet lamp, mercury-arc lamp, xenon lamp, tungsten-halogen lamp, metal-halide lamp, or a cold cathode fluorescent tube, it can decompose toxic substances, etc. more efficiently than can the silica gel of any conventional invention in which a titanium oxide film photocatalyst is fixed only on the surface of silica gel.
Also, since in the present invention silica gel carrying a titanium oxide photocatalyst not only in a particle form, but also in a powder form, is obtained, on the surface of which little titanium oxide is present, it can be incorporated in plastics, paints and varnishes, paper, etc., which are decomposable with titanium oxide, to make them have functions such as an antibacterial action, and functions for the deodorization and decomposition of dirt.
The present invention will be explained in detail by the following Examples.